Numeric checking

This is a discussion on Numeric checking within the C Programming forums, part of the General Programming Boards category; How can I validate a each character of an array to check if it is numeric? ie if (num[1] != ...

  1. #1
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    Question Numeric checking

    How can I validate a each character of an array to check if it is numeric? ie

    if (num[1] != 0)
    ......

    but I want to check against all digits, 0-9 using one line of code? I know there is a way like, [0-9], but cannot find it?

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    Me want cookie! Monster's Avatar
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    I don't want to use isdigit!

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    Registered User Vber's Avatar
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    ...

    Originally posted by spaceboo
    I don't want to use isdigit!
    And why that?
    so build your own one, that I think this should be very close to the isdigit one.

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    I've done something like this before using ascii values. As '0' has as ascii value of 48, and '9' has an ascii value of 57, then...

    Code:
    n = buf[x] - 48;
    if(    ( n < 10 )
      && ( n >= 0 ))
    then buf[x] is a digit.

    So get your string in a buffer, set up a for loop, and for each character in the string, do the above check.

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    I'm sure anything you make will be very close to isdigit(). Although if you have a real need of 'one line comparison' I suppose you can create a regexp in C and compare it to that. Heh.

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    Just Lurking Dave_Sinkula's Avatar
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    How can I validate a each character of an array to check if it is numeric? ie

    if (num[1] != 0)
    ......

    but I want to check against all digits, 0-9 using one line of code? I know there is a way like, [0-9], but cannot find it?
    If I were avoiding isdigit, I would do this.
    Code:
        if(num[1] >= '0' && num[1] <= '9')
            /* ... */
    Using '0' and '9' is more portable than using their ASCII equivalents 48 and 57 because it need not be ASCII.

    One "[0-9]" method is as follows.
    Code:
        char digit[2];
        if(sscanf(&num[1], "%1[0-9]", digit) == 1)
            /* ... */
    Although the %[0-9] is a common extension, it is non-standard. A standard version would be like this.
    Code:
        if(sscanf(&num[1], "%1[0123456789]", digit) == 1)
            /* ... */
    It meets the "one line" criteria (if you declare digit with num), but to me it seems quite excessive.

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